The present invention relates to a fuel cutoff system for an engine of a motor vehicle at the deceleration of the vehicle.
When a vehicle is decelerating, it is preferable to cutoff fuel so as to decrease fuel consumption. FIG. 3 shows a block diagram describing a conventional fuel cutoff system disclosed in Japanese Utility Model Laid-Open Nos. 52-99431 and 52-65720. The system comprises an accelerator switch, clutch switch, neutral switch provided in a transmission, other signal generators responsive to factors such as coolant temperature and engine speed, and a control circuit for controlling a fuel cutoff solenoid provided in an idle system in a carburetor or fuel injectors. When an accelerator pedal and a clutch pedal of a vehicle are released and one of the gears in the transmission is in engagement with another gear during deceleration, the above described switches are closed. Accordingly, the control circuit decides that the vehicle is in deceleration, and operates the fuel cutoff solenoid or fuel injectors to cutoff fuel.
However, when a failure occurs in the system, for example the failure in accelerator switch caused by sticking of a movable contact to a fixed contact or short circuit of a harness to the ground occurs, the result is the same as the release of the accelerator pedal in the normal operating condition of the accelerator switch. If the other switches are also closed, the control circuit decides that the vehicle is decelerating. Therefore, the fuel cutoff solenoid is operated to cutoff fuel in spite of the depression of the accelerator pedal, resulting in stall or surging of the engine, deterioration of driveability of the vehicle